DM ghl: D-block electronic configurations; transition metal catalysis

Question 1

Indicate the orbital blocks on the periodic table.

Answer 1

Question 2

Draw a diagram showing the energies of electron sub-shells from n = 1 to n = 4.

State the elements for which this diagram is correct.

Answer 2

Correct up to Ni in period 4, after which 4s sub-shell has higher energy than 3d.

Note: energy gaps should decrease with increasing principal quantum numbers, as below.

Question 3

What defines d-block elements?

Answer 3

Elements for which the last-placed electron is in a d-orbital.

Not outermost shell; 4s already filled since lower energy.

Question 4

What is the electronic configuration of argon?

Answer 4

1s²2s²2p⁶3s²3p⁶

Question 5

Give the electronic configuration of all of the elements in period 4 of the d-block, in terms of the electronic configuration of argon.

Answer 5

Question 6

Give the electronic configuration of chromium, and explain why it is is not [Ar]3d⁴4s2, as might be expected.

Answer 6

[Ar]3d⁵4s¹.

This is an energetically favourable arrangement since it avoids repulsion of two electrons in 4s orbital.

Question 7

Give the electronic configuration of copper, and explain why it is is not [Ar]3d⁹4s², as might be expected.

Answer 7

[Ar]3d¹⁰4s¹

This is an energetically favourable arrangement, since 2 electrons in the fifth 3d orbital experience less repulsion than 2 electrons in the 4s orbital.

Question 8

Draw a diagram showing the arrangement of electrons in the ground state of elements in the first row of the d-block.

Answer 8

Question 9

What is a transition metal?

Answer 9

A d-block element which forms one or more stable ions with incompletely filled d-orbitals.

Question 10

From where are electrons first lost when transition metals form simple ions?

Answer 10

The 4s sub-shell (and then the 3d sub-shell).

When empty, 3d has more energy than 4s, but this reverses when electrons are populated (quantum physics…)

Question 11

Which elements are transition metals?

Answer 11

Ti to Cu (not Sc or Zn).

All trans metals are d-block elements but not all d-block elements are trans metals.

Question 12

Use the electronic configurations of the common ions of zinc and scandium to demonstrate why they are not transition metals.

Answer 12

• Zn [Ar] 3d¹⁰4s² →* Zn²⁺ [Ar] 3d¹⁰
• Sc [Ar] 3d¹4s² →* Sc³⁺ [Ar]

Neither form stable ions with incompletely filled d-orbitals, so they are not transition metals.

Question 13

Why do transition metals exist in variable oxidation states?

Answer 13

There are several stable arrangements of their 3d and 4s electrons.

Question 14

• What are the common oxidation states of iron?
• Write their electronic configurations.

Answer 14

Fe [Ar] 3d⁶4s²

Oxidation state = +2: Fe(II) or Fe²⁺ [Ar] 3d⁶

Oxidation state = +3: Fe(III) or Fe³⁺ [Ar] 3d⁵

Question 15

With reference to electronic configuration, suggest and explain why Fe(II) is spontaneously oxidised to Fe(III).

Answer 15

• Fe²⁺ has 2 electrons in its 5th d-orbital, causing repulsion
• Fe³⁺ has only one electron in each d-orbital, so experiences less repulsion
• Fe³⁺ has the more energetically favourable configuration so is stabler

Question 16

• What are the common oxidation states of copper?
• Write their electronic configurations.

Answer 16

Cu [Ar] 3d¹⁰4s¹

Oxidation state = +1: Cu(I) or Cu⁺ [Ar] 3d¹⁰

Oxidation state = +2: Cu(II) or Cu²⁺ [Ar] 3d⁹

Question 17

What is the full electronic configuration of vanadium(III)?

Answer 17

V [Ar] 3d³4s²

V³⁺ 1s²2s²2p⁶3s²3p⁶3d²

Question 18

Why do the transition metals tend to have higher oxidation states only in combination with other elements?

Answer 18

• Higher oxidation states mean removal of more electrons
• Makes ions smaller + more charged
• High charge density
• More reactive
• More likely to occur in molecules

Question 19

Suggest why manganese(II) is more stable than manganese(III).

Answer 19

Mn²⁺ [Ar] 3d⁵

Mn³⁺ [Ar] 3d⁴

3d⁵ electronic arrangement is more stable than 3d⁴

Question 20

Answer 20

C

Question 21

What is a catalyst?

Answer 21

A chemical which increases the rate of a reaction by providing an alternative pathway with a lower activation enthalpy, but remains chemically unchanged.

Question 22

What is heterogeneous catalysis?

Answer 22

Catalysis in which the catalyst and reactants are in different physical states.

Question 23

What is homogeneous catalysis?

Answer 23

Catalysis in which the catalyst and reactants are in the same physical state.

Question 24

Why are 3d transition metals good:

• Heterogeneous catalysts?
• Homogeneous catalysts?

Answer 24

Heterogeneous: They have 3d + 4s electrons available to form weak bonds with reactants by adsorption.

Homogeneous: They exist in variable oxidation states, since there are several stable arrangements of their 4s + 3d electrons. This allows them to undergo redox.

Question 25

Describe the typical action of transition metals as heterogeneous catalysts.

Answer 25

• Gaseous/liquid reactant(s) are adsorbed onto solid metal surface by forming weak bonds with its 3d + 4s electrons
• Weakens + breaks bonds in reactants
• New bonds form
• Weak bonds break; products are released

Question 26

How does homogeneous catalysis usually occur for transition metal catalysts?

Answer 26

• Aqueous phase
• Metal ion + reactant(s) → intermediate → product(s)

Question 27

1. Write an overall equation for the reactions below.
2. Suggest what is acting as a catalyst, and justify your suggestion.
3. How could this hypothesis be tested?

Answer 27

1. S₂O₈^2-(aq) + 2I^-(aq) → 2SO₄^2-(aq) + I₂(aq)
2. Fe²⁺ since it is reformed (it is a reactant and final product)
3. Attempt reaction with and without Fe²⁺ present. If reaction is slower or does not occur without Fe²⁺, it acts as a catalyst.

You could record initial [Fe²⁺], then measure final [Fe²⁺] by manganate(VII) titration. If [Fe²⁺] were unchanged, you’d have proved it isn’t used up, but not that it’s involved in the reaction.

Question 28

An example of homogeneous catalysis is the reaction between 2,3-dihydroxybutanedioate ions and hydrogen peroxide, catalysed by Co(II) ions, which are pink in aqueous solution.

The reactant mixture begins pink, then turns green and back to pink. Suggest why.

Answer 28

• Mixture initially contains Co²⁺, which is pink
• Co²⁺ reduces H₂O₂ and is oxidised to intermediate containing Co³⁺, which is green
• Co²⁺ is reformed in products, so pink is seen again

Question 29

An example of homogeneous catalysis is the exothermic reaction between 2,3-dihydroxybutanedioate ions and hydrogen peroxide, catalysed by Co(II) ions.

Draw an enthalpy profile, showing both the uncatalysed and catalysed reaction.

Answer 29